“We are standing on the ground that could change the future of energy,” says engineer Laurent Pattison, deep in the reactor pit of the world’s biggest nuclear fusion project.

Around him is a vast construction site, all aimed at creating temperatures of 150mC on this spot and finally bringing the power of the sun down to Earth. The €18bn (£14.3bn) Iter project, now rising fast from the ground under the bright blue skies of Provence, France, is the first capable of achieving a critical breakthrough: getting more energy out of the intense fusion reactions than is put in.

“It is a bet that is very important for humanity,” says Johannes Schwemmer, the director of Fusion for Energy, the EU partner in the international Iter collaboration. “We need to get this energy once and for all.”

The long allure of nuclear fusion is simple: clean, safe, limitless energy for a world that will soon house 10bn energy-hungry citizens. But despite 60 years of research and billions of dollars, the results to date are also simple: it has not delivered.

Fusion is in danger of following its atomic cousin, conventional fission nuclear power, in over-promising – “electricity too cheap to meter” – and under-delivering. The Iter project itself, which stems from a cold war Reagan-Gorbachev summit in 1985, has seen years of turmoil. The US pulled out entirely between 1998-2003 and in 2008, Iter had to treble its budget and shift its deadline back a decade.

But leaders representing half the world’s population – through the Iter partners, the EU, China, Russia, US, India, Japan and South Korea – are now making the €18bn wager that fusion can deliver and have radically overhauled Iter’s management to fix the mistakes of the past.

The goal is to trap a plasma in a huge magnetic ring and force heavy hydrogen isotopes to fuse together to release prodigious amounts of energy – four times more than the splitting of uranium atoms produces in conventional fission reactors.

“We are convinced we can deliver hundreds of megawatts through Iter,” up to 10 times more energy than is put in, says David Campbell, the director of science and operations at Iter (which means “the way” in Latin).

To achieve that breakthrough, Iter will use a donut-shaped magnetic cage called a tokamak to trap the plasma. More than 200 smaller tokamaks have been built around the world and Campbell says the decades of physics and engineering that Iter is building on is a strong guarantee of success.

But nothing has ever been attempted on the scale of Iter. The world record for fusion power – 16MW - was set in 1997 at the JET reactor in the UK. The longest fusion run – six minutes and 30 seconds – was achieved at France’s Tore Supra in 2003. Iter is aiming for 500MW and 50-minute runs.

Once finished the reactor building will weigh 320,000 tonnes, three times more than the Eiffel Tower. Photograph: Iter

The site is a cathedral to the fusion dream: it spans the equivalent of 60 football fields and the reactor building will weigh 320,000 tonnes, all resting on rubber bearings in case of an unlikely, but not impossible, earthquake. The reactor itself will weigh 23,000 tonnes, three times more than the Eiffel Tower. It is the most complex engineering project in history.

More than 2,800 tonnes of superconducting magnets, some heavier than a jumbo jet, will be connected by 200km of superconducting cables, all kept at -269C by the world’s largest cryogenic plant, which will pump 12,000 litres per hour of liquid helium.

Millions of precision components will be shipped in from the seven partners to be assembled by thousands of workers. This is all aimed at keeping just two grammes of plasma hot enough and stable enough in the 30m-diameter tokamak for fusion to take place.

Iter’s schedule is to create the first plasma in 2025, then start firing tiny 5mm frozen pellets of heavy hydrogen – deuterium and tritium – into the plasma and generating energy. Deuterium is easily refined from seawater and fuses with tritium, which is harvested from fission reactors but could be self-generated in Iter in future. The aim is to reach its maximum power output by 2035 and, if so, Iter will be the foundation of the first fusion power plants.

Bernard Bigot, the director general of Iter, is certain it will produce plentiful power, “but what is not granted so far is that this technology will be simple and efficient enough that it could be industrialised,” he says.

The point of Iter is finding out, says Bigot: “The world needs to know if this technology is available or not. Fusion could help deliver the energy supplies of the world for a very long time, maybe forever.”

Even if things go well, getting real fusion power plants online before 2050 would be a triumph, raising an awkward question: what if fusion comes too late? Climate change is driving an accelerating transformation to low-carbon energy and drastic cuts in emissions are needed by 2050. If these are achieved, will there be a need for fusion power, which will be expensive at the start?

“It is certainly not going to be too cheap to meter,” says Campbell. But it’s a question of timescale, he says: “In the long term there are very few available options: renewables, fission and fusion.”

For Schwemmer, there is only one long-term option. “You would have to cover whole continents with wind turbines to produce the energy needed for 10 billion people,” he says. “And if our children’s children are not to sit on piles of [fission] nuclear waste, we have to make fusion work. Even if it takes till 2100, we should still do it.” Nuclear fission is also limited by uranium supplies, perhaps to a few decades if it were to play a large role.

Bigot said: “People have to realise, if we want a breakthrough [that could provide energy] for millions of years, 10 or 20 years is nothing.” He thinks fusion may still come in time to meet the need to move the world to zero emissions in the second half of the century to defeat global warming.

As a nuclear technology, some will remain implacably opposed to fusion. While fusion reactions produce only harmless helium, the high-energy neutrons also ejected irradiate the walls of the reactor, leading to radioactive waste.

A European contractor creates the inlets in the conductors for helium. An average of eight helium inlets will be needed per coil. Photograph: Iter

Again, the key is timescale, says Campbell. Waste from fission can remain radioactive for 250,000 years, making plans to store dangerous waste for many times longer than the whole of human civilisation speculative. In contrast, fusion waste will decay on the scale of decades. “Looking after the waste for 100 years is credible,” he says.

Fusion is also intrinsically safe, with the large meltdowns seen in fission accidents such as Fukushima and Chernobyl physically impossible. Part of the reason is the tiny amount of fuel in a fusion reactor at any one time and part is the temperamental nature of plasma, a boiling gas of ions and electrons. “If you lose control of the plasma, it doesn’t just sit there, it disappears like that,” says Campbell, clicking his fingers.

“After Fukushima, we thought we would be flushed down the toilet like all nuclear,” says Sabina Griffith, a communications manager at Iter. “But the opposite happened – governments thought if not fission, then what?”

There are other fusion reactor designs that might be better and, in particular, smaller. A €1bn reactor opened in Germany by chancellor Angela Merkel earlier in 2016 uses a stellarator, in which the plasma ring is shaped like a Mobius strip. This makes it potentially more stable and, crucially, able to operate continuously, rather than in pulses like a tokamak.

“There are technology routes that might let you build something smaller – in principle,” says Campbell. But he says they either rely on unproven “exotic” ideas or underestimate the heavy engineering needed to contain burning plasmas. “Iter is the size our present technology allows us to build,” he says.

Politics remains a challenge to delivering Iter and uncertainty has been ramped up by the election of Donald Trump as president of the US, where some powerful voices want to leave the project for good. Britain’s vote to leave the EU has also added to the uncertainty.

But Bigot believes the need to know if full fusion power is feasible will keep the partners in. “To be frank, the US is only 9% of the project, if they were to leave alone, I believe we could go on,” he said. “But it would be the wrong signal [showing] the most powerful country in the world is not preparing for its future.” On Brexit he says: “It would damage Iter a little, but it would damage the UK a lot,” given its long and continuing research in fusion.

The Iter council includes representative from its seven members: China, India, Russia, the EU, Japan, US and South Korea. Photograph: Iter

The political problems usually boil down to costs and the governments of Iter partners wanting to reduce the taxpayers’ money spent on the project. “Iter looks very expensive to the ordinary person in the street,” says Campbell. “But the cost is spread across half the world’s population. Seen in that context I don’t think it is such a big investment to make.” The world spent $325bn on fossil fuel subsidies in 2015 alone, according to the IEA, and $150bn on renewable energy support.

Fusion supporters such as Campbell also suggest fusion has geopolitical benefits because its key fuel – heavy hydrogen – is accessible to all. “No one has a monopoly on the fuel so no one is going to fight each other over it.”

The 1985 Reagan-Gorbachev summit that kickstarted the Iter project called for “the widest practicable development of international collaboration” in nuclear fusion to obtain “energy which is essentially inexhaustible, for the benefit of all mankind”.

So how far is the world from achieving that, 30 years and numerous stumbles on? Many still point to the answer given by Lev Artsimovich, the father of the tokamak and head of the Soviet fusion power programme for more than two decades until his death in 1973. Fusion power, he said, will arrive “when mankind needs it – maybe a short time before that”.

• This article was amended on 2 December 2016 to correct General Fusion’s location.